(1) Field of the Invention
The present invention relates to the continuous casting of steel and particularly to the problem of steel reoxidation. In particular, the invention relates to a tundish comprising an assembly comprising a nozzle and a surrounding refractory element preventing or limiting steel reoxidation, and to the assembly itself. According to other of its aspects, the invention also relates to such a surrounding refractory element and to a continuous steel casting process.
With growing demands for quality and property control, cleanliness of steel becomes more and more important. Issues like controlling the chemical composition and the homogeneity have been supplanted by concerns generated by the presence of non-metallic inclusions. Especially the presence of aluminium oxide inclusions is considered as harmful both for the production process itself as for the steel properties. These inclusions are mainly formed during the deoxidation of the steel in the ladle, which is necessary for continuous casting. Incomplete removal of the non-metallic inclusions during secondary metallurgy and reoxidation of the steel melt cause nozzle clogging during continuous casting. The layer of clogged material contains generally large clusters of aluminium oxide. Its thickness is related to the amount of steel cast as well as to the cleanliness of the steel. Nozzle clogging results in a decreased productivity, because less steel can be cast per unit of time (as result of the decreasing diameter) and due to replacement of nozzles with concurrent casting interruptions. Besides clogging, the presence of reoxidation products may give rise to erosion of the nozzle and to the formation of inclusion defects in the steel.
(2) Description of Related Art
Several solutions have been developed in the art to prevent steel reoxidation. In particular, the molten metal stream is generally shrouded with a pouring shroud during its transfer from a casting vessel to a downstream vessel (or mold) to prevent direct contact between the poured steel and the surrounding atmosphere. Argon is often injected directly at the surface of a pouring nozzle so as to shield the molten metal stream. The surface of the steel melt in a metallurgical vessel (for example a tundish) is generally covered with a liquid slag layer so as to prevent direct contact between the steel and the surrounding atmosphere. Alternatively (or in addition), the atmosphere above the tundish can be made inert (use of oxygen scavenger or of inert gas such as argon).
Further solutions have been developed in the art to remove non-metallic inclusions and reoxidation products when they are present in the tundish. These solutions consist generally in facilitating the floatation of these inclusions and reoxidation products so that these are captured by the floating slag layer. For example, dams, weirs, baffles and/or impact pads can be used to deflect upwardly the molten metal stream in the tundish. Inert gas bubbling device can also be used to float out inclusions and reoxidation products.
Other solutions also exist for making the inclusions and oxidation product harmless. For example calcium based alloys can be used to eliminate some of the problems generated by the presence of aluminum oxide inclusions.
All these prior art solutions have contributed to improve the general cleanliness of the steel but have not yet permitted the casting of inclusion—or reoxidation products—free steel. Moreover, some of the prior art solutions can, in turn, generate new defects in the steel (such as gas bubbling, calcium-based alloy), can be expensive (use of inert atmosphere) or environmentally unacceptable. For these reasons, it would be desirable to propose an alternative solution which would solve the above problem, which would be economical and would not raise environmental problems.